A cable modem termination system (CMTS) is communication equipment typically located in a cable operator's headend facility. The CMTS provides high-speed data services, such as cable Internet or voice over Internet Protocol (VoIP), to customer locations. An example of a CMTS is the Motorola Broadband Service Router 64000 (BSR 64000).
A hybrid fiber-coaxial (HFC) network is a broadband network that combines optical fiber and coaxial cable to provide two-way communication between the CMTS and a cable modem or multimedia terminal adapter (MTA). The cable modem is a communication device located at the customer location that receives communication signals from the CMTS on downstream channels, and transmits the other communication signals to the CMTS on upstream channels. The MTA is a communication device at the customer location that provides both the functionality of a cable modem, and VoIP telephone service communication.
Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable television system. Channel bonding is a DOCSIS 3.0 feature that enables a cable modem at a customer location to use multiple downstream channels, or multiple upstream channels, together at the same time. For example, a cable modem configured with four upstream channels can use DOCSIS 3.0 channel bonding to increase the throughput of the upstream communication with the CMTS. The cable modem distributes, or segments, the data packets among the four channels in an upstream bonding group and transmits the data packets to the CMTS in parallel, rather than in series.
The “DOCSIS 3.0 MAC and Upper Layer Protocols Interface Specification” defines the “T4 timeout” parameter as the time that a cable modem will wait for unicast ranging opportunity. In addition, the “DOCSIS 3.0 MAC and Upper Layer Protocols Interface Specification” states that “In Multiple Transmit Channel Mode the CMTS MAY increase the value of the T4 timeout by means of the T4 Timeout Multiplier in order to reduce CMTS overhead associated with scheduling RNG-REQ slots and processing RNG-RSP messages.” Thus, if a CMTS vendor chooses to implement the T4 Timeout Multiplier, they may schedule the ranging messages between the CMTS and the cable modems less frequently when operating in upstream channel bonding mode.
Customer locations typically always power-on a cable modem to provide the customer with instant access to the Internet, and in the case of an MTA, also to ensure that the customer's telephone service is always available. Anytime the cable modem is powered-on, whether idle or transmitting data, it must maintain registration with the CMTS and participate in the calculation of the upstream channel quality metric, such as a digital modulation quality metric like the modulation error ratio (MER), or a power-based signal quality metric like the signal-to-noise ratio (SNR or S/N). When the cable modem is not transmitting data, it relies solely on ranging messages to maintain registration and for the calculation of the upstream channel quality measurements.
In an exemplary prior art system, a CMTS configured to use DOCSIS 3.0 upstream channel bonding may communicate with a DOCSIS 3.0 bonded cable modem configured with the DOCSIS T4 Timeout Multiplier. If the cable modem uses, for example, four upstream channels in a bonding group and a ranging interval for each upstream channel of 10 seconds, for example, the T4 Timeout Multiplier is set to 4 (i.e., the number of bonded channels) and the ranging interval increases to 40 seconds (4×10 seconds). However, the CMTS also periodically monitors the quality of the upstream channels by retrieving the upstream channel SNR measurements, for example, every 10 seconds, especially when the cable modem includes a VoIP adapter. Since the T4 Timeout Multiplier increases the ranging interval, when the cable modem is not transmitting data, and since the calculation of the SNR measurements relies solely on the ranging messages, the use of the T4 Timeout Multiplier can affect the accuracy of the SNR measurements. Regardless of the approach taken, the prior art CMTS that implements the T4 Timeout Multiplier suggested by DOCSIS to reduce the ranging traffic can create a problem with the channel quality measurements. The following two examples illustrate the possible impact on the accuracy of the SNR measurements.
In the first example, when the prior art CMTS monitors the upstream channel quality by retrieving the SNR measurements for the channel from a single modem, the cable modem will complete its first ranging exchange, and will wait 40 seconds (4×10 seconds) before the next ranging exchange. After the cable modem completes its first ranging exchange, the CMTS can measure the quality of the channel by retrieving the SNR statistics for the channel from its Broadcom registers, and clearing those registers to prepare for the next SNR measurement. These SNR statistics are valid. Since the CMTS monitors the quality of the upstream channels periodically, but before the next ranging interval, the next time the CMTS measures the quality of the channel, if the cable modem is idle during that period (“idle” meaning no ranging or data passing), the SNR statistics will be zero because the previous reading of the SNR statistics cleared the Broadcom registers that store the SNR data. This will continue until the CMTS measures the quality of the channel after the next ranging exchange. When the SNR statistics are zero, the CMTS is not able to determine the quality of the channel. Thus, in this example, the T4 Timeout Multiplier suggested by DOCSIS to reduce the ranging traffic causes a problem with the channel quality measurements.
In the second example, when the prior art CMTS monitors the upstream channel quality by averaging the SNR measurement for the channel from all the cable modems using the channel, the cable modems will complete their first ranging exchange, and will wait, for example, 40 seconds (4×10 seconds) before the next ranging exchange. After the cable modems complete their first ranging exchange, the CMTS measures the quality of the channel by retrieving the SNR statistics for the channel from its Broadcom registers, and clearing those registers to prepare for the next SNR measurement. These SNR statistics are valid. Since ranging exchanges occur, for example, every 40 seconds, and channel quality is monitored, for example, every 10 seconds, the first time the SNR statistics are retrieved following a ranging exchange the SNR statistics are valid, but SNR statistics for the channel on all of the modems cannot be trusted because not all the cable modems range at the same time. Laboratory experiments that included between 50 and 100 cable modems on an upstream channel that was unimpaired resulted in average channel SNR measurements of 42, 40, 23, 18, 38, and 42. The inconsistency of these SNR measurement will result in the prior art CMTS spectrum management services swapping frequencies or modulation profiles unnecessarily because it will think that the channel has gone from unimpaired to impaired and back to unimpaired.
Since the CMTS can reduce the frequency of the exchange of ranging messages, there is a need to maintain the stability of the upstream channel quality measurements in an upstream channel bonded system when the DOCSIS T4 Timeout Multiplier is in use.